1. Field of the Invention
The present invention relates to an image display apparatus, and more specifically, to an image display apparatus capable of cutting unnecessary light so as to be suitably used in a medical field.
2. Description of the Related Art
Various diagnostic image acquiring apparatuses utilizing X-rays etc. have conventionally been used in medical related fields. For instance, X-ray apparatuses, CR (computed radiography) apparatuses, CT (computed tomography) apparatuses, and MRI (magnetic resonance imaging) apparatuses are now in practical use.
Diagnostic image information acquired by these apparatuses is recorded on photographic films and is observed on a film viewer (Shaukasten). Alternatively, the diagnostic image information is recorded on other recording media, subjected to desired image processing such as frequency processing and gradation processing, and is displayed by image display apparatuses (monitor screens) for observation. In this manner, the diagnostic image information is used for diagnoses at medical care sites.
CRT display apparatuses have conventionally been used as the image display apparatuses, although they are recently replaced by so-called flat panel displays using liquid crystal panels or organic EL panels. Generally, these flat panel displays have various advantages over CRT display apparatuses. With such displays, for instance, installation spaces are saved, apparatus weights are reduced, and power consumptions are lowered. As a result, the flat panel displays are expected to become more and more widespread in various fields including the medical field.
It should be noted here that a CR (computed radiography) apparatus is a radiographic image recording and reading apparatus using an accumulative fluorescent material (stimulable phosphor) that operate as follows. First, radiation is emitted and a part of radiation energy is accumulated in the accumulative fluorescent material or the sutimulable phosphor. Then, excitation light such as visible rays or infrared light is emitted onto the accumulative fluorescent material or the stimulable phosphor, whereby the stimulated luminescence is exhibited according to the radiation energy accumulated therein. In the CR apparatus, first, radiographic image information of a subject, such as a human body, is recorded on the stimulable phosphor by an X-Ray apparatus and the like. Then, the radiographic image information represented by the stimulated luminescence is photoelectrically read to obtain image signals. Then, the thus obtained image signals are subjected to image processing. Finally, the thus processed image signals are output to display a soft copy image on an image display apparatus or to record a hardcopy image on a X-Ray photographic film.
Also, a CT (computed tomography) apparatus is based on a computed tomography method, with which projection images are obtained using parallel X-ray beams on straight lines from various angles and X-ray absorption coefficients of tissues in a human body, which represents the composition of the tissues, are obtained through computation of these data. The thus obtained composition of the tissues is output as a soft copy image or a hardcopy image. The tissue composition is reconstructed from the projection images using, for instance, a successive approximation method or an inverse-projection method.
Further, a usual MRI (magnetic resonance imaging) apparatus obtains an electromagnetic wave signal generated by a nuclear magnetic resonance effect of hydrogen atoms and converts the signal into an image. In more detail, an electromagnetic wave at a natural resonance frequency of nuclei is applied from the outside to place a nuclear magnetic moment due to spins in an excitation state. Then, the application of the electromagnetic wave is terminated under this condition to have the nuclei sequentially undergo a transition from the excitation state to a ground state. During this transition, an electromagnetic wave at a resonance frequency is emitted. This electromagnetic wave is received by coils and is converted into an image. The thus converted image is output as a softcopy image or a hardcopy image.
When a radiographic image is taken and recorded onto an x-ray film or a recording sheet (stimulable phosphor sheet) that uses the stated sutimulable phosphor in the radiographic image recording and reading apparatuses such as the CR apparatus, it is required to minimize the effect of radiation exposure on a living body and, for instance, to prevent the degradation of image quality due to scattered light resulting from light that is unnecessary for observation. Consequently, in many cases, a radiation field limiter made of lead or the like is used to limit a radiation field so that only necessary parts of a subject are exposed to radiation.
If image radiography is performed using the radiation field limiter, an image of a subject is recorded in a radiation field area of a recording medium such as an X-ray film or a stimulable phosphor sheet. Here, the radiation field area refers to an area of the recording medium within the outline of an opening of the radiation field limiter. On the other hand, radiation does not reach an area (outside area radiation field) of the recording medium outside the opening outline and therefore the extra-radiation field area remains in an unexposed state. Accordingly, the outline of the radiation field area (hereinafter, a “radiation field outline”) corresponds to the opening outline and becomes an edge line.
FIG. 8A shows a X-ray image taking apparatus using a radiation field limiter, while FIG. 8B shows a stimulable phosphor sheet on which is formed a radiation field outline PS of an image that corresponds to the opening outline of the radiation field limiter. As shown in FIG. 8A, the radiation field limiter, which is made of a lead plate and has a rectangular opening, is arranged between an X-ray source and a subject. A portion of the radiation field limiter outside the opening prevents X-rays from reaching unnecessary parts of the subject and the stimulable phosphor sheet.
When the X-ray source emits X-rays toward the subject under this condition, X-rays passing through the subject strikes the stimulable phosphor sheet. During the emission of the X-rays, as shown in FIG. 8B, the outside area of the radiation field (the extra-radiation field area) Pout outside the opening of the radiation field limiter is not exposed to the X-rays, while the X-rays reach the area (the radiation field area) Pin within the opening and therefore X-ray information of the subject is recorded in this area. It should be noted that the X-ray image information of the subject is also recorded on an X-ray photographic film in the same way as the case of the stimulable phosphor sheet shown in FIG. 8B.
If the recording medium, on which an image has been recorded only within the radiation field area Pin, is directly observed (in case of the recording medium being an X-ray photographic film, for instance), or if an image generated by processing information read from such a recording medium with the radiographic image reading apparatus is observed (in case of the recording medium being a stimulable phosphor sheet, for instance), the extra-radiation field area of the recording medium is in an unexposed state and therefore does not bear any images. Consequently, the outside light passes through the extra-radiation field area during observation of the image and the observation is severely hindered by the passing light.
In more detail, if a plurality of X-ray photographic films are placed on a film viewer (Schaukasten) for observation or if a plurality of images are electrically displayed on a screen of an image display apparatus, the extra-radiation field area allows unnecessary light to enter into the eyes of an observer (mainly, a doctor). This is a great hindrance to observation of the images.
In view of this problem, the assignee of this patent application proposes a medical image display system in JP, 11-276441, A. In this system, LCDs (liquid crystal display apparatuses) are used to display images. Sensors for measuring the brightness of the outside light are provided in the vicinities of the display surfaces of the LCDs and the illumination of the backlight of each LCD is controlled according to the measurement results of these sensors.
This system is highly effective at controlling the illumination of each LCD, although not being effective for the light resulting from the stated extra-radiation field area. The problem discussed in this specification is to cut unnecessary light within an image display surface as much as possible.